Magnetic resonance imaging of hyperpolarized (HP) noble gases is an excellent tool for pulmonary imaging. For optimization of imaging strategies, such as projection encoding or rapid gradient-recalled echo sequences, used to acquire morphologic images of the pulmonary gas space, knowledge of the relaxation times in the lung in vivo under varying physiologic conditions is crucial. The dependence of 3He and 129Xe longitudinal relaxation rates upon the concentration molecular oxygen in noble gas/O2 mixtures is well known. Assuming that dipolar coupling with paramagnetic O2 is the dominating contribution to HP noble gas T1 in lung gas spaces, determination of T1 would further provide a non-invasive technique for measuring the partial pressure of oxygen (PO2) in the lung in vivo. Purpose of the project is therefore to measure 3He and 129Xe T1 in the guinea pig lung in vivo at varying oxygen levels during periods of suspended breathing. First experiments will be performed with non-localized spectroscopy techniques. Based upon these results, single-shot imaging techniques, such as spiral projection encoding, will be developed for further extension toward maps of the regional T1 respectively PO2.